Two mutilated ferrets have been found in a letter box in Tasmania's north-west, prompting an animal cruelty investigation.

A woman who lives near Smithton told police the animals were still alive when she found them.

She took them to a vet where they were put down.

Peter West from the RSPCA says it was not certain where the ferrets came from.

"It's very serious, this sort of stuff is so sad - and for the ferrets involved. They are feeling creatures and certainly don't deserve this sort of treatment," he said.

"The RSPCA and Tas Police are calling on the public, especially those up on the north-west coast of Tasmania who may know something about any missing ferrets or anything at all about this case specifically to call Crimestoppers or call the RSPCA animal cruelty hotline."